The present invention generally relates to a method for measuring a heat transfer coefficient which is necessary to measure in order to determine the properties of many kinds of fluid by the so-called electrical heating method. The present invention also relates to a sensor which can measure a heat transfer coefficient by a heat transfer element.
The term "fluid" as used in the present invention includes not only a liquid substance and a gaseous substance but also a semi-solid substance, namely a substance which can flow.
Generally, it is very important for a process controll of a fluid to measure properties of the fluid (for example kinematic viscosity).
In Japanese Patent laid-open application No. 60 (1985)-152,943, there is disclosed a method for measuring properties of a fluid by using a thin metal wire as a heat transfer element by putting the thin metal wire into a fluid as a measuring object, charging the thin metal wire with electricity so as to the heat transfer element and then calculating a heat transfer coefficient on the surface of the thin metal wire.
The abovementioned method is well known as shown in FIG. 6. That is to say, a conduction lead wire 2--2 and a voltage measuring lead wire 3--3 are connected at the two ends of a thin metal wire 1 which is a heat transfer element made of a heating material. An electric current is sent into the thin metal wire 1 through the conduction lead wire 2--2 and the voltage applied to the thin metal wire 1 is measured by a volt meter 4 connected with the voltage measuring lead wire 3--3. According to the relation of the voltage V measured by the volt meter 4 to the electric current I of the thin metal wire 1, a electrical resistance R is calculated and further a calorific value W is calculated by the following formula (1) EQU W=I.sup.2 R (1)
A heat transfer coefficient .alpha. at the boundary surface between the thin metal wire and the fluid is calculated by using the calorific value W and the following formula (2) EQU .alpha.=w'd/4 (.theta..sub.s -.theta..infin.) (2)
d: diameter of thin wire PA0 w': W/v (volume of thin wire) PA0 .theta..sub.s : surface temperature of thin wire PA0 .theta..infin.: temperature of fluid
A kinematic viscosity is calculated by the heat transfer coefficient .alpha. according as the well-known relational expression which, for example, appears in the Japanese reference (Japan Food Industry Academy Review, 1988 vol. 1 .sctn. the introduction.
In the abovementioned method for measuring a heat transfer coefficient of a fluid, the heat being transfered in a non-radial direction from the two ends 5, 5 will be unknown. However, when the heat transfer element is made of a thin wire (a ratio of diameter to length of the thin wire is about &lt;1:1000), the heat loss abovementioned from the two ends 5, 5 becomes extremely small as compared with that lost from the circumferential surface of the thin metal wire 1. Therefore, when disregarding the heat capacity lost from the two ends 5, 5 and regarding the heat capacity W of the whole heat transfer element as a heat capacity escaping to the fluid from the circumferential surface, the measurment error thereof is small.
However, when it is desired to miniaturize the heat transfer element, the heat transfer coefficient between the heat transfer element and a measuring object cannot be exactly judged. That is a new problem to be solved.
That is to say, as shown in FIG. 7, when a metal stick 6, formed by shortening the thin wire, is used as a heat transfer element and the circumferential surface 7 thereof is contacted with a fluid as the measuring object, in the abovementioned method for measuring the heat transfer coefficient, the desired heat capacity is a heat capacity transfered between the heat transfer element and the measuring object, namely, it corresponds to a heat capacity W.sub.1 escaped from the surface 7 of the metal stick to the fluid.
However, in a miniaturized heat transfer element, the heat generated from the heat transfer element 6 also escapes from the two end surfaces 8, 8 of the heat transfer element 6 as shown in the figure. When a heat capacity lost from the two end surfaces 8, 8 is W.sub.2, the whole calorific value of the heat transfer element 5 is the sum of the heat capacity W.sub.2 and the heat capacity W.sub.1 transferring to the fluid. EQU W=W.sub.1 +W.sub.2 ( 3)
In the miniaturized heat transfer element, the ratio of W.sub.1 /W.sub.2 is not essentially infinite, which is different from that of the method for measuring a heat transfer coefficient of a fluid by using the long thin metal wire, and the unmeasured heat capacity W.sub.2 becomes larger in comparison with the heat capacity W.sub.1. The difference thereof cannot be ignored.
Accordingly, in the above case, when regarding the heat capacity W of the whole heat transfer element as the heat transfer element W.sub.1 transferring into the surrounding fluid, the heat transfer coefficient cannot be exactly measured because of the error resulting from the assumption that no heat capacity W.sub.2 is lost from the two end surfaces 8, 8.
These disadvantage occurs similarly, when using a heat absorption material as a heat transfer element.